Several hundreds of years before Europeans arrived, the area that became Okanogan County was home to numerous indigenous peoples that would eventually become part of three Indian reservations referred to as the Northern Okanogans or Sinkaietk, Tokoratums, Kartars and Konkonelps. They spoke in seven types of Interior Salish languages related to the Puget Sound tribes. The Okanogans experienced a favorable climate, having camped in the winter, hunting bears in the spring, catch fish in the summer and hunt deer in fall. The camps consisted of teepee-like longhouses built with hides and bark. A popular destination for this was the Kettle Falls also situated in Washington where the Columbia River dropped over 20 feet (6.1 m). Meanwhile, women gathered several pieces of nuts, roots and berries.

Due to its remoteness, the area that became Okanogan County was one of the last in Washington settled by white people. It was an early thoroughfare used by prospectors to gain access to other communities, some of which contain gold fields in what is now known as British Columbia, a province in Western Canada. By the 21st century, the region specialized in agriculture, forestry and tourism. Electric producer Grand Coulee Dam was constructed between 1933 and 1942, originally with two power plants, around the Okanogan and Grant counties at the former's southern border.[6]

In July 2014, the Carlton Complexwildfire burned over 250,000 acres (390 sq mi; 1,000 km2) in Okanogan County. It destroyed over 300 homes[7] including 100 in and around Pateros.[8]

According to the U.S. Census Bureau, the county has a total area of 5,315 square miles (13,770 km2), of which 5,268 square miles (13,640 km2) is land and 47 square miles (120 km2) (0.9%) is water.[9] It is the largest county in the state by area, and it is larger than three states in land area.

There were 15,027 households out of which 33.20% had children under the age of 18 living with them, 54.40% were married couples living together, 11.00% had a female householder with no husband present, and 29.60% were non-families. 24.50% of all households were made up of individuals and 9.70% had someone living alone who was 65 years of age or older. The average household size was 2.58 and the average family size was 3.04.

In the county, the population was spread out with 27.70% under the age of 18, 7.30% from 18 to 24, 25.50% from 25 to 44, 25.50% from 45 to 64, and 14.00% who were 65 years of age or older. The median age was 38 years. For every 100 females there were 99.20 males. For every 100 females age 18 and over, there were 98.00 males.

The median income for a household in the county was $29,726, and the median income for a family was $35,012. Males had a median income of $29,495 versus $22,005 for females. The per capita income for the county was $14,900. About 16.00% of families and 21.30% of the population were below the poverty line, including 28.20% of those under age 18 and 10.40% of those age 65 or over.

As of the 2010 United States Census, there were 41,120 people, 16,519 households, and 10,914 families residing in the county.[16] The population density was 7.8 inhabitants per square mile (3.0/km2). There were 22,245 housing units at an average density of 4.2 per square mile (1.6/km2).[17] The racial makeup of the county was 73.9% white, 11.4% American Indian, 0.6% Asian, 0.4% black or African American, 0.1% Pacific islander, 10.1% from other races, and 3.5% from two or more races. Those of Hispanic or Latino origin made up 17.6% of the population.[16] In terms of ancestry, 21.4% were German, 12.4% were Irish, 12.2% were English, and 3.6% were American.[18]

Of the 16,519 households, 29.4% had children under the age of 18 living with them, 49.7% were married couples living together, 10.7% had a female householder with no husband present, 33.9% were non-families, and 28.0% of all households were made up of individuals. The average household size was 2.45 and the average family size was 2.96. The median age was 42.9 years.[16]

The median income for a household in the county was $38,551 and the median income for a family was $48,418. Males had a median income of $37,960 versus $29,032 for females. The per capita income for the county was $20,093. About 14.7% of families and 19.5% of the population were below the poverty line, including 27.3% of those under age 18 and 9.2% of those age 65 or over.[19]